This invention relates generally to radar and, more specifically, to terrain imaging radars.
Systems that use radar to extract information about the surroundings have been used for many years. These include systems that provide range measurement, altimetry, angle measurements, synthetic aperture radar (SAR) imagery for the horizontal extent of a terrain/surface and three-dimensional interferometric SAR terrain mapping.
Systems that provide terrain contour mapping have been used for moving platforms such as airplanes and missiles. For example, U.S. Pat. No. 5,170,171 incorporated herein by reference in its entirety, describes a system for three-dimensional interferometric SAR terrain mapping employing altitude measurement. In such systems, SAR data is used in combination with separately generated altimeter data to produce a terrain map corrected for aircraft roll angle. In general, such systems use two synthetic radar antennas for transmission at shallow grazing angles and a downward pointed ranging altimeter located on the airborne platform.
Another system that is able to provide a terrain map is disclosed in U.S. Pat. No. 5,260,708, also incorporated herein by reference in its entirety. In this system, a three-dimensional interferometric SAR terrain-mapping system is described in which unambiguous phase unwrapping employing subset bandwidth processing is used. As with the system of the ""171 patent, this system employs two radar receivers which process echo signals conventionally to yield slant range and Doppler frequency data for plural resolution cells. The measured phase difference for each resolution cell provides an ambiguous measure of slant range difference to the two antennas needed to determine terrain elevation and correct ground range. The received echo data is reprocessed using less than the entire bandwidth of the radar transmission to achieve additional center wavelengths. This produces a differing ambiguity interval and permits unambiguous determination of the slant range difference. The average altitude is separately determined by a ranging altimeter.
Other topographical mapping radar systems, such as disclosed in U.S. Pat. No. 4,359,732, incorporated herein by reference in its entirety, use two vertically spaced fan beam antennas to provide terrain contour mapping. This radar transmission is normal to the flight path and to the side of the airborne platform.
A radar apparatus that looks downward in a synthetic aperture, interferometric mode is disclosed in U.S. Pat. No. 5,867,119, also incorporated herein by reference in its entirety. This system is designed to provide precision height measurements and does not provide images of the scene below the radar.
Although such systems have been used in mapping terrain, their accuracy is somewhat limited by the accuracy of the separately generated altimeter information. They also typically provide mapping at a considerable distance to the side of the airborne platform, thereby requiring significant power. High-powered radar signals are undesirable emissions from military aircraft and missiles because they are easily detectable by enemy surveillance equipment. Therefore, such signals must not be constantly onxe2x80x94that is, have a duty cycle of 1. Therefore, a need exists for reducing the power of terrain mapping radar signals in order to make airborne platforms and missiles more resistant to enemy systems. There also exists a need to provide more accurate and useful altimeter information.
The present invention is a system for performing surface imaging of the surface below an airborne platform. The system includes an antenna that transmits a radar signal and receives radar return information from one or more directions directly below the airborne platform to an angular direction of approximately 30 degrees forward of straight down. The system also includes a processor that generates surface map information based on the received radar return information. The generated surface map information includes down-track information, cross-track information, and range information relating to the airborne platform""s position relative to the surface. Because the signals are pointed downward, less power is required to transmit the radar signals. Because less power is required, detection by enemy systems is more difficult. This allows the radar of the present invention to always be on or on more than in the past.
In accordance with further aspects of the invention, an image processor generates an image based on the down-track, cross-track, and range information.
In accordance with other aspects of the invention, a guidance system generates guidance information based on the down-track, cross-track, range information, and a previously stored reference map.
The present invention is preferably related to a radar apparatus used on an airborne platform to provide information about the surroundings of the platform. In general, the invention relates to a system that transmits and receives electromagnetic energy, modulates the transmitted energy and extracts information from the received energy to establish knowledge of its physical surroundings in three-dimensional space. In particular, the invention relates to a system that transmits energy in a generally downward direction and provides precise three-dimensional pixel location of each element of the surface and a three-dimensional image of the scene below or up to 30 degrees forward of the platform. The system then compares the three-dimensional image information to stored data to provide automatic guidance.